py: Use a wrapper to explicitly check self argument of builtin methods.
Previous to this patch a call such as list.append(1, 2) would lead to a seg fault. This is because list.append is a builtin method and the first argument to such methods is always assumed to have the correct type. Now, when a builtin method is extracted like this it is wrapped in a checker object which checks the the type of the first argument before calling the builtin function. This feature is contrelled by MICROPY_BUILTIN_METHOD_CHECK_SELF_ARG and is enabled by default. See issue #1216.